There are many things you can do to find a charity PR consultant or agency - a simple Google search on charity PR will bring up various consultants and agencies that work with the charity sector. Or a search in Yellow Pages will bring up a list of local PR consultants.
It's always worth trying to get a recommendation - why not chat to other charities to find out whether they have used a PR consultant and what their experience was?
There are various things you can do to shortlist consultants (3-5 is a good number to aim for). Most PR consultants have a website, which enables you to research their approach, the services they offer, and the other clients they have worked with.
Decide whether you want to work with a freelance consultant/small agency, or a large agency. Unless you are a very large charity, freelance consultants and small agencies are probably the best option to ensure you are a highly-valued client who rates immediate attention. An agency with a multi-million pound turnover is likely to focus its most talented staff on large corporate clients that are investing vast sums into PR.
Look for a consultant that only has expertise in the charity sector, or a few sectors including the charity sector. Check out any testimonials on their website, in particular testimonials from charities. Why not call the charity listed in the testimonial to hear first hand what their experience was of working with that consultant?